"Whom" is used as the object of a verb or preposition. It is a pronoun that refers to a person or people.
Here are some examples:
* Correct: To whom did you give the book?
* Incorrect: To whome did you give the book?
* Correct: The woman whom I met is my neighbor.
* Incorrect: The woman whome I met is my neighbor.
If you are unsure whether to use "who" or "whom," try substituting "he" or "him" for the pronoun. If "he" works, use "who." If "him" works, use "whom."